This invention relates to a process for producing α-, β-unsaturated carboxylic acids from off-gas and waste gas streams.
The current commercial process to produce acrylic acid uses a two-step process that first converts propylene to acrolein and subsequently converts acrolein to acrylic acid. An absorber tower, which in part separates the residual off-gas from the liquid product, typically immediately follows the reactor(s). This off-gas, called absorber off-gas (AOG), has a wide range of potential compositions but primarily contains unreacted gases from the oxidation reaction.
High purity chemical or polymer grade propylene currently is used as the feedstock to the commercial process. The cost of propylene, and in particular high purity propylene, is projected to increase significantly in the future, and economic incentives exist to use lower grade propylene streams, e.g. from FCC units in an oil refinery, to produce acrylic acid. However, the use of propylene streams containing higher levels of undesired propane creates a problematic situation for the AOG composition, which normally is sent to be processed as waste gas, e.g. sent to a thermal oxidizer. Issues such as: (1) loss of C3 hydrocarbon to combustion, (2) generation of increased CO2 waste gas, and (3) overloading of the thermal oxidizer, present difficulties in the use of propane-containing propylene feeds.
In addition, propane in the chemical grade propylene will not react with conventional catalysts used in the commercial manufacture of acrylic acid from propylene. The propane, however, takes up space in the reaction zone thus reducing the overall capacity of the reactor. Therefore, although there is an economic advantage to using the lower grade of propylene, there is a corresponding loss of capacity equivalent to the propane content of the propylene. Commercial chemical grade propylene may have as much as 7% by weight propane so the resulting capacity loss can be significant in a world-class oxidation train.
It would be desirable to have a modified conventional propylene process for producing carboxylic acids that could operate using low-purity propylene feedstocks. Furthermore, it would advantageous if the modified conventional process could be capable of converting unreacted propane in the AOG to acrylic acid product to reduce the negative impact of the capacity loss.